The present invention relates to the art of receiver systems for receiving and demodulating modulated signals, and more particularly to an asynchronous system for recovering the signals modulating the in-phase, and quadrature-phase components of a modulated signal.
Many systems have been proposed for transmitting and receiving stereophonic signals in the AM frequency band. Several of these schemes have proposed modulating the two program signals onto differently phased carriers and then linearly combining them to form the composite stereo signal which is to be transmitted. If the phase angle between the two carrier signals is set at approximately 90.degree., the system represents a standard quadrature modulating system. Other systems have been proposed, however, wherein the phase angle between the two carriers is reduced to be less than 90.degree.. A system of this latter sort is disclosed in the co-pending application of Leitch, U.S. Ser. No. 812,657, filed July 5, 1977.
Synchronous detection schemes have generally been employed to demodulate signals which have been modulated in this fashion. Synchronous detectors include circuitry (usually a voltage controlled oscillator) for providing a reference phase signal, as well as a phase-locked-loop to maintain the phase of the reference signal in a specific phase relationship with the carrier component of the incoming signal. The inclusion of this circuitry, of course, adds to the cost and complexity of the receiver system.